1. Field of the Invention
The present invention relates to an aircraft position monitoring system for continuously monitoring the present positions of many aircraft.
2. Description of the Related Art
In the prior art, when the present positions of many aircraft during navigation are monitored by a ground station, the position data obtained by a radar is displayed on a display device. On the other hand, the position of an aircraft flying outside the range of a radar (i.e. on the sea) is monitored such that the pilot acquires position data periodically from a navigation system mounted in the aircraft, the data is delivered to the ground station via a voice communication system, and an air traffic controller receives the data and records the data.
Conventionally, in the voice communication system, electric waves of an HF band are reflected by the ionosphere. Although this technique is suitable for long-distance communication, a wide band cannot be obtained and S/N is low. Thus, the communication capacity is limited and the communication quality is not sufficient.
Because of the limitation to the communication capacity, the number of communication lines is limited and the control interval must be increased for communication with many aircraft. In particular, in the air traffic control communication for aircraft flying in an oceanic airspace, the repeating rate of position data reporting of an aircraft is about once per hour, and thus the traffic volume has to be limited. In addition, the problem of communication quality may prevent smooth communication between pilots and air traffic controllers and the traffic safety may not be maintained.
In order to solve the above problems and cope with the increasing traffic volume, while ensuring traffic safety, an ADS (Automatic Dependent Surveillance) system has been developed and is being standardized worldwide. Specifically, according to the ADS system, air/ground communication lines having wide coverage and high quality (e.g. by satellite communication) are employed, and position data is automatically transmitted from the aircraft to the ground station at short cycles. The aircraft position is displayed on a display device on the basis of the data received by the ground station. Thereby, the position of aircraft can be continuously monitored in real time.
In the ADS system which is proposed at present, it is considered to use, as data to be transmitted from an aircraft to the ground station, three-dimensional aircraft position data and identification (ID) data (aircraft ID), as well as data relating to the time of position data acquisition, the velocity of the aircraft, the course of the aircraft, the wind, the temperature, etc. These data items are generally called ADS data or ADS message. Further, data relating to position determination precision and presence/absence of redundancy in the case of providing a plurality of navigation systems is included in the ADS data (this is called "FOM" (Figure of Merit)).
As a navigation system with which an aircraft can detect its own position, there are generally known an INS (Inertial Navigation System) and a GNSS (Global Navigation Satellite System); typically, GPS (Global Positioning System).
In the meantime, ADS data transmitted automatically from many aircraft must be edited and processed at the ground station by a desired communication line and a computer, thereby displaying positions of aircraft which can easily be recognized by controllers or others. Although there have been proposed various display modes, it is suitable to employ a display mode similar to radar display which has long been used in air traffic control fields. Such radar display shows an image or symbol indicating a two-dimensional position of the aircraft, as well as the type of aircraft, altitude, and other necessary data.
According to the radar display mode, the position and ID data is derived from the ADS data transmitted periodically from the aircraft, the data is collated with prepared navigation schedule data, and the data is edited for easy recognition on the basis of the collation result. However, it is troublesome to perform editing each time the data is transmitted, and the data processing amount of the system may be limited. Furthermore, when the aircraft position control is to be performed by using the ADS data including the FOM data, the data processing will become more troublesome.
There is a simpler method in which the tracking processing technique used in the air traffic control radar is employed. In the tracking processing technique, on the basis of a preceding position track of a target (aircraft), a subsequent position range of the target is estimated and the position data in the estimated range is considered as the data of the same target.
When the tracking processing technique is applied to a navigation position monitoring system using ADS data, the FOM data must be taken into account in order to set an optimal estimated range. The processing for setting the estimated range is very troublesome and difficult, and the data processing amount of the system increases.
In order to simplify the tracking processing, the estimated range may be determined irrespective of the FOM data. In this case, however, if the estimated range is too narrow, the tracking processing cannot be started smoothly when the target is initially captured or the navigation system is switched for some reason. Moreover, if the estimated range is too large, the position precision of the tracking processing is lost.
As has been stated above, it has been strongly desired that an aircraft position monitoring system be constructed by effectively utilizing FOM data, thereby realizing simple tracking processing and real-time, continuous aircraft navigation, while preventing an increase in data processing amount.